Purifying zinc metal



'-.March l2, 1935.

P. MCL. GINDER ErAL PURIFYING Z INC METAL Filed April 28, 1935 W INVENTORS y l m wig/MQW,

ATTORNEYS Parened M 12, 193s UNITED STA-Tas.,

PATENT o1-Fics Philip McLean Ginder, Willis McGerald Peirce,

and Robert Kerr Waring, Palmerton, Pa., assignors to The New Jersey' Zinc Company, New York, N. I., a corporation of NewJersey Applieausaprn 2s, 1933, serial No. 668,314

' 5 (o1. '1s-2s) out this specication and the appended claims,

lead and cadmium are taken as representative of metal impurities of higher and lower boiling points, respectively, than zinc, and it is therefore'to be understood that specific mention of lead and cadmium in the specification and claims is illustrative and not restrictive.

In our copending application for Letters Patent of the United States, Serial No. 626,634, filed July 2, 1932, we have disclosed a method of and apparatus for removing cadmium and the like from zinc metal by fractional distillation in a rectifying column. In that application we have described a novel and advantageous combination of our method of cadmium elimination withV the method of eliminating lead from zinc'metal by the rectifying action of a reux column described in the copending application of Messrs. Holstein and Ginder, Ser. No. 540,566, led May 28, 1931. This application for patent and the invention thereof involve this conjoint application of rectification treatments for purifying zinc metal of both lead and cadmium (or the like) in two suc- 30 cessive treatment operations. Thus, the present invention involves subjecting vapor derived from impure zinc metal contaminated with lead and cadmium to two successive rectification treatments in the first of which a vapor purified of lead is obtained and delivered to the second rectification treatment from which molten zinc metal purified of cadmium is obtained.

'I'he invention may be carried out in variousforms and types of apparatus. We have illustrated in the accompanying drawing an apparatus adapted for the practice of the invention. In the drawing,

Fig. 1 is a sectional elevation of the apparatus, and

Figs. 2, 3 and 4 are detailed views of the front end of the zinc distillation retort.

Referring to the drawing, there is shown a retort 10 for volatilizing the impure zinc metal, mounted in an appropriate heating furnace of conventional construction. Molten zinc metal contaminated with lead and cadmium (or other metal impurities of higher and lower boiling temperatures than zinc) is charged into the retort through an inclined pipe 11 extending through the front wall of the retort and having an upturned discharge end resting on the bottom 'of the retort '(Figs. 2, 3 and 4).' Theouter end of thevpipe 11 is normally closed with a plug 12,

metal is to be charged into the retort. This parwhich is replaced by a funnel 13 -when molten ticular `type of charging device is of advantage with zinc metal containing relatively high percentages lo1' iron, where undue cooling vof the molten metal in the charging device should be avoided. l Any other appropriate means for charging molten metal, either intermittently or continuously, into the retort may be used. A normally closed `tap hole 14 in the bottom of the '.irontv wall of the retort permits .the withdrawal therefrom of lead-*high residues when necessary. The retort 10 is connected by an elbow pipe 15 to the bottom of a relix or rectifying column 16 Aoperating to remove lead and other metal impurities with boiling points higher than that of zinc from zinc vapor ascending the column. The rectifying column 16 is built up of superposed rectangular trays of silicon carbide or other appropriate refractory material. 'Ihe top and bottom edges of the trays are bevelled as at 17 so th'at they may be stacked one above the other to form the column. The bottom of each tray has a transverse opening 18 approximate one end thereof. The upper surface of each tray is provided with transverse ribs 19 parallel to the slot 18; the ribs being of progressively increased height from the end nearest the slot to the end furthest from the slot. The trays 16 are stacked in staggered relationship with respect to the slots 18 so that they form a column of superposed baiiiesthrough which metallic vapor ascends and molten metal descends in tortuous or zig-zag paths. The ribs 19 serve to retain shallow pools of molten' metal on the trays. The cascade effect of molten metal pouring over the ribs and'falling from tray to tray facilitates intlmate contact between ascending vapor and descending molten metal.

'Ihe built-up rectifying column 16 is surrounded by appropriate heat insulation 20, such as refuse zinc oxide, held in place by a steel casing 21. The assembled column may be supported in any vappropriate manner. A heatinsulated pipe 22 conducts zincvapor (puried with respect` to lead) from the top of the lead-eliminating column 16 to a point intermediate the top and bottom of the cadmium-eliminating column 23.

The cadmi -eliminating rectifying column 23v is built up of superposed trays of the same general type as employed in the lead-eliminating column, the trays being however of somewhat ,lower part of the rectifying column 23 is sur'v rounded for an appropriate length thereoi! by a combustion chamber or heating laboratory? 24 provided with a fuel gas burner 25 and a flue 26 for the escape of exhaust combustion gases. The base or lower end of the column 23 communicates through a conduit 27 with a molten metal sump 28, provided with a tap hole 29 and externally heated by a combustion chamber 30 having a burner 31 and a stack 32 for the escape of exhaust combustion gases.

Above the combustion chamber 24, the rectifying column 23 is surrounded by an appropriate layer of heat insulating material 33. The relative lengths of the column 23 surrounded by the combustion chamber 24 and by the heat insulation 33, respectively, may be varied to meet different operating conditions encountered in practice.

The rectifying column 23 is surmounted by a dephlegmator 34. The amount of condensation in the dephlegmator is controlled by heat insula- 'tion in appropriate amount. Molten metal condensing in the dephlegmator is deiiected by an inclined baille 35 into a well 36 from which the molten metal overflows into the top tray of the rectifying column. A skimming bar 37 holds back (from the molten metal overflowing the well 36) any oating metallic dust (blue powder or the like), and such 'floating metallic dust is removed from time to time through a normally plugged tap hole 38 in the wall of the dephlegmator.

The top of the dephlegmator 34 is connected by a pipe 39 to a canister 40 for condensing and collecting (in the form of metallic dust) the vapor escaping from the top of the dephlegmator. A

gas line 41, provided with clean-out plugs 42,

connects the canister 40 with a gas reservoir 43. 'Ihe gas reservoir consists of a cylindrical lower casing 44 within which vertically slides a cylindrical bell or top casing 45; the annular space between the two casings being provided with a water seal 46. The bell 45 is counterpoised by a countterweight 47 connected to the bell by a cable 48.

In practicing the present invention in the apparatus illustrated in the drawing, metallic Japor, derived by distillation from the impure zinc metal in the retort 10, flows upwardly through the leadeliminating rectifying column 16 in contact with downwardly iiowing (reuxing) molten metal. Throughout the height of the rectifying column 16, the ascending vapor is progressively freed of lead while the descending molten metal is progressively enriched in lead. On leaving the top of the rectifying column 16, the metallic vapor has been substantially deprived of all contaminants (such as lead) with boiling points exceeding that of zinc. metalic vapor is generally cadmium. y

'Ihe metallic vapor (puriiied with respect to lead) enters the cadmium-eliminating rectifying column 23 through the pipe 22 at a point between the upper heat insulated portion of the column and the lower heated portion of the column. The firing of the combustion chambers 24 and 30 is so adjusted and regulated as to rectify continuously the mixture of zinc and cadium entering the column through the pipe 22.A In this manner, the ascending vapor is progressively enriched in cadmium and the descending (rei'luxing) molten metal is progressively freed of cadmium. A cadmium rich zinc vapor escapes from the top of the column into the dephlegmator 34, while reiined zinc metal (that is zinc metal with greatly de- 'I'he chief remaining impurity in the creased cadmium content) is tapped from the sump 28 through the tap hole 29 at appropriate intervals.

All but a cadmium-rich fraction of the vapor entering the dephlegmator 34 is condensed therein 5 and is returned to the rectlfying column23 for the further removal of cadmium therefrom. The cadmium-rich fraction of thevapor uncondensed in the dephlegmator escapes through the pipe 39 to the canister 40 where it condenses to a cad- 10 mium-rich dust.

The gas reservoir 43 is iilled with nitrogen or equivalent inert gas, and serves to compensate for changes in gas conditions within the entire system tending to raise or lower the operating 15 gas pressure therein. Thus, when the gas pressure within the system decreases for any reason (such, for example, as a decrease in the rate of volatilization oi.' zinc in the retort 10), the gas and vapor within the canister tend to contract 20 in volume on account of cooling. The reduced pressure resulting from such contraction in gasl volume would tend to permit air to enter through the outlet of the system, if it were not for the supply of nitrogen, or other appropriate inert 25` gas, that is then supplied to the canister from the gas reservoir. When the canister again heats up. gas is forced back into the reservoir.

Moltenpzinc metal (of the grade to be refined) is charged from time to time (or continuously 30 if desired) into the retort 10 to maintain an appropriately uniform volume of metal in the retort. The retort is continuously operated until the lead content of the molten metal therein is increased to such a high percentage that the 35 vapor delivered from the top of the rectifying column 1 6 to the rectifying column 23 has as high a lead content as can be tolerated in the puriied zinc metal. The retort is then cut out of operation, is cleaned of the lead-high metal resi- 40 due therein, and is then recharged with molten zinc metal of the grade undergoing purification.

The apparatus illustrated in the drawing as adapted for the practice of the invention may be variously modified in construction and ar- 45 rangement of operating units without departing from the principles of the invention. In general, the improved apparatus of the invention comprises a cadmium-eliminating rectifying column operatively associated with a lead-eliminating 50 rectifying column so that vapor purified with respect to lead is delivered from the top of the lead-eliminating rectifying column to the cadmium-eliminating rectifying column. Various other forms of transverse bames may be used in place of the trays illustrated in the drawing. For simplicity of description, a single operating unit is shown in the drawing, whereas in commercial practice, it will generally be desirable to embody in the apparatus a plurality or a battery of distillation retorts with associated lead rectifying columns in conjunction with one or'more cadmium rectifying columns in order to approximate substantial continuity of operation and uniformity of purified zinc product.

We have herein described the redistillation of impure zinc metal as the most usual source of impure zinc vapor for the practice of the invention. It will be understood however that zinc vapor containing lead and cadmium may be derived in various other ways .and from other sources, as for example directly from a zinc smelting or reducing operation.

We claim:

1. The method of purifying zinc metal contaminated with lead and cadmium which comprises subjecting metallic vapor derived from the impure zinc metal to two successive rectification treatments in the iirst of which a vapor purified of lead is obtained and delivered to the second rectiiication treatment from which molten zinc metal purified of cadmium is obtained, and discharging metallic vapor from said second rectiiication treatment into a condenser which communicates with a reservoir of inert gas.

2. An apparatus for purifying zinc metal containing lead and cadmium comprising a retort for volatilizing the zinc metal, a rectiying column operatively associated with said retort, means permitting the ow oi vapor from. said retort to the lower end of said rectifying column and the countercurrent iiow ci' molten metal from the rectifying column back into the retort, a second rectifying column, means for conveying vapor from the topv of said first rectifying column to said second rectifying column, means permitting the withdrawal of puried zinc metal from the base of said second rectifying column, means for withdrawing cadmium-rich vapor from the top of said second rectifying column, and a reservoir for inert gas connected with the vapor discharge outlet of said second rectifying column.

3. A method of purifying zinc metal contaminated by lead and cadmium which comprises volatilizing the zinc metal by heating it in a retort by conduction through a Wall of said retort, passing the resulting vapor in a substantially undiluted condition into a rectifying column in contact with molten metal resulting from condensation of said vapor, owing molten zinc containing the lead back into the retort for further volatilizationf withdrawing zinc vapor purified of lead but still contaminated with cadmium from the top of said rectifying column, delivering the vapor puriiied of lead in the vapor state into a second rectifying column, passing the vapor purified of lead in the second rectifying column in contact with molten metal resulting from condensation of the vapor purified of lead, heating the base of the second rectifying column byconduction to volatilize in part molten zinc owing thereto, withdrawing molten zinc puried of cadmium from the base of said second rectifying column, and withdrawing cadmium-enriched metal from the top of the sec'- ond rectifying column.

t. Apparatus for purifying zinc metal containing lead and cadmium, comprising a retort for volatilizing zinc metal, means for heating the zinc metal by conduction through a wall of the retort, means i'or charging zinc metal containing lead and cadmium into said retort below the level ci the zinc metal contained therein, a rectifying column operatively associated with said retort, means for transferring the vapor from said retort to the lower end of said rectifying column and the countercurrent flow of molten metal from the rectifying column back into the retort, a second rectifying column, means for conveying vapor from the top of said rst rectifying column to said second rectifying column, means to Withdraw purified zinc metal from the base of said second rectifying column, and means to withdraw cadmium-rich metal from the top of the second rectifying column.

5. An apparatus for purifying zinc metal containing lead and cadmium comprising a retort for volatilizing the zinc metal, means for heating the zinc metal by conduction through a wall of the retort, a rectifying column operatively associated with said retort, means for transferring vapor from said retort to the lower end of said rectifying column and the countercurrent ow of molten metal from the rectifying column back into the retort, a second rectifylng column, means for heating said second rectifying column by conduction through a wall thereof, a conduit for conveying vapor from the top of said iirst rectifying column to said second rectifying column, means to withdraw purified zinc metal from the base of said second rectifying column, and means to withdraw cadmium-rich metal from the top of the second rectifying column.

1 MCLEAN GINDER.

S MCG PmCE.

ROBERT l l. v 1 WARING. 

